This invention relates to electrical capacitors, and more particularly to monolithic capacitors made by lamination and firing of electroded dielectric layers.
Multilayer monolithic capacitors comprise a multiple number of dielectric layers, at least some of which bear metallizations (electrodes) in desired patterns. Such capacitors are made from green (unfired) tape of ceramic particles held together with an organic binder, by cutting pieces of tape from a sheet of tape, metallizing some of the tape pieces, stacking and laminating the pieces of tape, and firing the resultant laminate to drive off organic binders and any solvents and form a sintered (coherent) body, which is termed monolithic. Rodrieguez et al. U.S. Pat. No. 3,456,313 discloses a process for making them. FIG. 1 of Fabricius U.S. Pat. No. 3,223,905 shows a multilayer capacitor, which may be of alternating palladium and barium titanate layers.
Metallizations useful in producing conductors for multilayer capacitors normally comprise finely divided metal particles, applied to dielectric substrates in the form of a dispersion of such particles in an inert liquid vehicle.
Monolithic multilayer capacitors are typically manufactured by co-firing barium titanate formulations and conductive electrode materials in oxidizing atmospheres at temperatures of 1200.degree.-1400.degree. C. This process yields durable, well-sintered capacitors with high dielectric constant, e.g., greater than 1000. However, firing under these conditions requires an electrode material with high melting point, good oxidation resistance at elevated temperatures, sinterability at the maturing temperature of the dielectric, and minimal tendency to interact with the dielectric at the sintering temperature. These requirements normally limit the choice of electrode materials to the noble metals platinum and palladium, or to alloys of platinum, palladium, and gold.
Significant savings in electrode costs could be realized if dielectric materials could be modified to (1) yield good dielectric properties (high dielectric constant and low dissipation factor) after firing in reducing atmospheres, so that base metals could be used as electrodes, and/or (2) sinter at temperatures of 950.degree. C. or lower so that silver, which is significantly less costly than the other noble metals but has a lower melting point (962.degree. C.), could be used in electrode formation.
Attempts have been made to modify barium titanate ceramics so that they may be fired in reducing (e.g., hydrogen) or inert (e.g., argon, nitrogen) atmospheres. The use of this approach has been somewhat limited in that the electrical properties, e.g., dielectric constant, dissipation factor, temperature coefficient of capacitance, etc., are compromised as compared with those of conventional air-fired compositions. In addition, maintaining an inert or reducing atmosphere involves an additional production cost as compared to firing in air. Exemplary of this approach is Buehler U.S. Pat. No. 3,757,177, disclosing capacitors of base metal electrodes (e.g., Ni, Co, Fe) and modified barium titanate (MnO.sub.2, Fe.sub.2 O.sub.3, CeO.sub.2, CaZrO.sub.3) fired in an inert atmosphere at about 1300.degree. C. (col. 3, lines 33-34). Even with these high firing temperatures and the expense of firing in an inert atmosphere, the highest dielectric constant reported there is 1800 (col. 3, line 67).
Several attempts have been made to reduce the maturing temperature of dielectrics by mixing high temperature ferroelectric phases (titanates, zirconates, etc.) with glasses which mature at relatively low temperatures. Examples of this approach are given in Maher U.S. Pat. No. 3,619,220; Burn U.S. Pat. No. 3,638,084; Maher U.S. Pat. No. 3,682,766; and Maher U.S. Pat. No. 3,811,937. The drawback of this technique is that the dilution effect of the glass often causes the dielectric constant of the mixture to be relatively low, in the 25-200 range.
Another technique for lowering the sintering temperature of titanate-based dielectrics is by the use of "sintering aids." Additions of bismuth oxide or bentonite to barium titanate lowers the maturing temperature to about 1200.degree. C. (Nelson et al. U.S. Pat. No. 2,908,579). Maturing temperatures of 1200.degree.-1290.degree. C. may be attained by addition of phosphates to titanates as described in Thurnauer et al. U.S. Pat. No. 2,626,220. However, in each of these cases, the decrease in maturing temperature is not sufficient to permit the use of co-fired silver electrodes and dielectric properties are often degraded.
There exists a need for a composition which can produce a high dielectric constant (e.g., 1000 or above) and low dissipation factor (e.g., less than 5%, preferably less than 3%) and sinters in air at low temperatures (e.g., less than 1000.degree. C. or even 950.degree. C. or less). This would permit co-firing with silver or palladium/silver electrodes and hence would greatly reduce the cost of high dielectric constant multilayer capacitors.
N. N. Krainik et al. (Soviet Physics-Solid State, 2, 63-65, 1960), report solid solutions between, inter alia, PbTiO.sub.3, and PbMg.sub.0.5 W.sub.0.5 O.sub.3. Apparently a wide range of compositions, with 0-80% PbTiO.sub.3, was investigated (see FIG. 2). Firing was carried out in an atmosphere of PbO vapor, which precludes practical commercial applicability. No suggestion was made as to the manufacture of multilayer capacitors. In a second article from the same laboratory, G. A. Smolenskii et al. (Soviet Physics-Solid State 3, 714, 1961) report investigating certain solid solutions, including those of Krainik et al. Firing was similarly done in PbO. Phase transitions are discussed. In what is apparently a third article in this series, A. I. Zaslavskii et al. (Soviet Physics-Crystallography 7, 577, 1963), X-ray structural studies are reported.
Brixner U.S. Pat. No. 3,472,777 discloses the manufacture of ferroelectric ceramic discs by a two step firing process. Each firing step is taught to occur in the range 800.degree.-1200.degree. C., in air. In the sole example firing was at 1050.degree. C. Brixner discloses various dielectric compositions such as PbMg.sub.1/3 Ti.sub.1/3 W.sub.1/3 O.sub.3, Pb.sub.0.8-0.9 Sr.sub.0.1-0.2 Mg.sub.1/3 Ti.sub.1/3 W.sub.1/3 O.sub.3 and Y-containing compositions.
Incorporated by reference herein is Sheard U.S. Pat. No. 3,872,360, issued Mar. 18, 1975, relevant to the preparation of monolithic multilayer capacitors.